The invention relates to an apparatus for coating board-shaped articles, especially printed circuit boards.
After they have been provided with conductors, printed circuit boards are coated with a protective layer. The protective layer is a preferably UV-hardenable photographically exposable plastics material, lacquer or the like. After drying of the protective layer, it is exposed in a suitable form and developed at the exposed sites. The conductors are then free at those sites and can be electrically contacted. In the case of printed circuit boards that are provided with conductors on both sides, the second surface of the boards is also coated and treated analogously to the first surface. The printed circuit boards are coated in a coating installation which comprises a series of processing stations through which the printed circuit boards to be coated pass in succession.
The previously cleaned printed circuit boards are transported to the entrance of a coating station in which one side of the printed circuit boards is provided with a protective layer. The coating station comprises a pouring table with a free-falling pouring curtain under which the printed circuit boards are transported with the surface to be coated uppermost. If desired or necessary, a preheating station is arranged upstream of the coating station. There the previously cleaned printed circuit boards are preheated in a stream of hot air before being coated. From the preheating station, which is generally constructed as a circulating oven, the printed circuit boards are transferred to the coating station. When the first side has been coated, the printed circuit boards are transported to a vapour-removal and drying station following on from the coating station. In the vapour-removal and drying station, hot air is passed over the coated surface of the printed circuit boards. In that manner, solvent contained in the coating is evaporated (removed by a current of air) and suctioned off, and the coated surface is at least partially dried. In the case of the known coating installations, the printed circuit boards are also turned in the vapour-removal and drying station so that the coated surface of the printed circuit board is facing downwards when it reaches the exit of the station. If the second side of the printed circuit boards is also to be coated, the printed circuit boards are again transported under a pouring curtain of a coating station and finally subjected to vapour-removal and drying in a vapour-removal and drying station.
A coating installation construct ed for coating printed circuit boards on both sides is described, for example, in U.S. Pat. No. 4,926,789. In this known coating installation, a second coating station and a second vapour-removal and drying station follow on from the first coating station and the first vapour-removal and drying station. The printed circuit boards turned in the first vapour-removal and drying station are transported with the as yet uncoated surface uppermost under the free-falling pouring curtain of the second coating station. Then, in the second vapour-removal and drying station, they are transported through one or two circulating vapour-removal driers arranged one behind the other. Hot air is again conveyed over the coated surfaces in order to evaporate (remove by a current of air) solvent contained in the coating, the solvent is removed by suction and the printed circuit boards are dried.
U.S. Pat. No. 5,113,785 and U.S. Pat. No. 5,113,701 describe coating installations that permit printed circuit boards to be coated on one or both sides, as desired. The described coating install stations comprise only a single coating station and a single vapour-removal and drying station. If the printed circuit boards are to be coated on both sides, then, after being turned, they are again transported under the pouring curtain of the first coating station and then subjected to vapour-removal and drying in the single vapour-removal and drying station. In the case of the installation described in U.S. Pat. No. 5,113,785, the coating station and the vapour-removal and drying station are arranged side by side in the longitudinal direction of the installation. Arranged upstream and downstream of the coating station and the vapour-removal and drying station are transverse conveyors which transfer the printed circuit boards from the transport line with the coating station to the adjacent transport line with the vapour-removal and drying station. The coating installation described in U.S. Pat. No. 5,113,701 is constructed in a single line. In that installation, the vapour-removal and drying station is arranged substantially above the coating station. The printed circuit boards which have been coated on one or both sides are subjected to vapour-removal and dried while they are being transported in specially constructed holding devices above the coating station back to the entrance to the coating station or to the exit of the coating installation. Apart from the requirement for special holding means for the printed circuit boards, that coating installation has a large overall height because the vapour-removal and drying station is arranged substantially above the coating station.
While the described coating installations of the prior art meet the requirements of modern printed circuit board coating technology, they are nevertheless in need of improvement in one respect or another. It is a common feature of all three coating installations described that they require relatively great expenditure with respect to the infrastructure at the installation site. The work of installation is considerable because the preheating station and the vapour-removal and drying station(s) must each be connected into the air supply and removal system on site. It is especially to be borne in mind that the exhaust air of the vapour-removal and drying station(s) generally contains flammable solvent vapours and the installations must therefore meet safety standards. The solvent-containing exhaust air must be fed to the removal devices present on site where the solvent vapours are burnt together with other exhaust air or are removed from the exhaust air by some other method.
Pumps must be provided in order to suck in the inlet air and in order to circulate the air in each vapour-removal and drying station and, if present, in the preheating station. The air supplied to the preheating station and, especially, to the vapour-removal and drying station(s) has to be heated. For that purpose, heating devices are to be provided for the individual stations, which devices heat the inlet air to the desired temperature. The arrangement of the suction and circulating pumps and the heating device inside the preheating station and especially inside each vapour-removal drier considerably increases the space requirement of those devices and, in particular, their overall height is often too great and gives rise to problems during the transport and mounting of those devices.
The known coating installations for printed circuit boards have to be installed at sites that meet certain minimum standards in respect of clean-room quality, the intention being to prevent dirt or dust particles from reaching the surfaces of the printed circuit boards to be coated and impairing the quality of the coating. Dirt or dust particles that have settled on the surface can act as scattering centres during the subsequent photographic exposure of the coating, so that the coating is exposed, and then developed, at undesired sites. In the worst case, the contaminants on the surfaces can even result in short-circuits between the conductors. The printed circuit board affected is unusable in each case. In order to achieve the required clean-room quality, great expenditure is required with respect to the purifying and filtering of the air volume in the rooms in which the coating installation is installed. Bearing in mind the space requirement of coating installations, especially those for coating printed circuit boards on both sides, and the necessary size of the rooms in which the coating installations accordingly have to be set up, it is easy to understand why the provision and maintenance of the necessary clean-room quality take up a not inconsiderable proportion of the operating costs of the coating installation.